The self-assembly process to obtain an ordered array of patterns on a surface, also known as nanopatterning, is one method that can execute utmost precision. However, despite the exceptional precision and accuracy, these self-assembly nanopatterning methods have often lacked effectiveness over a large area.
However, a team of Researchers from Italy have now created a hierarchically-ordered polymethylmethacrylate (PMMA) nanofiber and nanodot array patterned over a large surface area, using a fast, easy and low-cost method named ASB-SANS.
The realization of nanopatterns onto a surface has many applications and is of great importance across the nanotechnology sector. The approach of top-down self-assembly has graced the nanotechnology community for many years, but it’s starting to meet its limitations. This has never been truer, as the emergence of advanced bottom-up techniques has enabled many surfaces to be patterned (and coated) with a much higher efficiency and precision.
One of the most promising bottom-up approaches, is that of self-assembled monolayers (SAMs). The utilization of SAMs has shown to allow a uniform modification towards the chemical nature of the surface across a large surface area. However, to bring this technique up to a high precision, accuracy and organization, a form of soft-lithographic support is required to aid in the specific SAM deposition across a substrates surface.
The Researchers from Italy have recently developed a new method known as Auxiliary Solvent-Based-Sublimation Aided NanoStructuring (ABS-SANS). It is a wet-processing method capable of producing nanopatterns over large areas in a short-time frame and is effective on both polymers and carbon nanotubes, without any additional pattern-directing techniques. The method uses a ternary solution (TS), a target material (TM), an Auxiliary Solvent (AS) and a Sublimating Substance (SS).
The Researchers have now utilized this method and performed an extensive study to see how a change in the composition of the ternary solution affects the ASB-SANS-generated PMMA nanopatterns in terms of size and topology. Scanning electron microscopy (SEM) was used to image the topology of the patterned surface.
The Researchers found that simple changes to the ternary solution allowed them to control the transition from continuous nanofibers to aligned nanodots, through a variety of intermediate topologies. The Researchers also found that the width of the dots/fibers and their inter-pattern distances varied due to the ratio of the solution components and was documented through a ternary diagram. The research has suggested that the macromolecular chains mobility follows Zimm-like models, with fibers and dots aligning with a lateral width.
The Researchers also found that the generated PMMA nanodot arrays could be used as lithographic masks for silicon substrates in Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) methods. The nanopillar masks were found to produce high aspect ratio silicon nanopillars over areas of several thousands of μm2.
The size and relative distance between the patterns was found to be an exponential decay function of the relative concentration of the auxiliary solvent in the ternary solution. This has suggested that the self-assembly process occurs within the sublimating substance matrix and can be described using common polymer chain diffusion laws. However, this is currently something which is under verification by the Researchers.
It has also been proposed that the sublimating substance crystallites induce an effective templating effect. It has also been thought that the transition from the continuous fiber alignment to the nanodot arrangement is determined by thickness of sublimation substance left on the target material after the auxiliary solvent has evaporated.
The ternary diagram derived by the Researchers will act as a guide for rational design of future PMMA patterns over large areas, and will aid in a better understanding of the behavior of the formulated ternary solutions. The practical applications of ASB-SANS processes in the real-world could incorporate a wide variety of applications, including photon trapping in photovoltaic cells, surface-enhanced sensors and plasmonics.
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“Controlled self-organization of polymer nanopatterns over large areas”- Eryilmaz I. H., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-09463-z